This invention relates to a process for making a lithographic printing plate, more specifically to a process for making a lithographic printing plate using an aluminum plate as a support and utilizing a silver complex diffusion transfer process. With regard to a lithographic printing plate using a silver complex diffusion transfer process (the DTR method), some examples are described on pages 101 to 130 of Andre Rott and Edith Weyde, "Photographic Silver Halide Diffusion Processes", published by the Focal Press, London and New York (1972).
As described therein, there have been known two kinds of lithographic printing plates using the DTR process, i.e., a two sheet type in which a transfer material and an image-receiving material are separated and a mono-sheet type in which these materials are provided on one support. The two-sheet type lithographic printing plate is described in detail in Japanese Provisional Patent Publication No. 158844/1982. Also, the mono-sheet type is described in detail in U.S. Pat. No. 3,728,114.
The mono-sheet type lithographic printing plate utilizing the silver complex diffusion transfer process with the use of an aluminum plate as a support (hereinafter referred to as "an aluminum lithographic printing plate") is described in detail in Japanese Provisional Patent Publications No. 118244/1982, No. 158844/1982, No. 260491/1988, No. 116151/1991 and No. 282295/1992, and U.S. Pat. Nos. 4,567,131 and 5,427,889.
In the above-mentioned aluminum lithographic printing plate, physical development nuclei are carried on a roughened and anodized aluminum support, and a silver halide emulsion layer is further provided thereon. A general process for making this lithographic printing plate comprises the steps of exposure, DTR development processing, water washing processing (washing off: removal of a silver halide emulsion layer with washing solution) and finishing processing.
More specifically, a metal silver image portion is formed on the physical development nuclei by the development processing and the silver halide emulsion layer is removed by the subsequent washing processing to expose the metal silver image portion (hereinafter referred to as a "silver image portion") Simultaneously, the anodized aluminum surface itself is exposed as a non-image portion.
After the washing processing, a finishing solution containing a protective colloid such as gum arabic, dextrin, carboxymethyl cellulose, polystyrenesulfonic acid, etc. is coated on the exposed silver image portion and the non-image portion for protecting the same. A processing of the so-called gum coating is applied to the surface of the plate. The finishing solution is also called as a fixing solution and the solution generally contains a compound which makes the silver image portion oleophilic such as a nitrogen-containing heterocyclic compound having a mercapto group or a thione group.
In the above-mentioned aluminum lithographic printing plate, there is a problem that ink removable property is bad at the time of starting printing. In the field of the offset rotary press printer, a printing method which has been used is that an etching by dampening solution is omitted in many cases at the time of starting printing to prevent lack of paper, and after attaching ink on the whole surface of the plate, dampening solution is fed and the ink at the non-image portion is removed. In such a printing, delay in an ink-removing rate at the non-image portion markedly lowers printing efficiency.
Moreover, as a problem in connection with the non-image portion, there is a stain due to a fingerprint. This is a problem that ink is adhered to the fingerprint attached at the non-image portion of the plate surface at the tie of handling the printing plate at printing to cause stain.
Also, as a problem regarding the non-image portion, there is a problem of fine line reproducibility. This is a problem that a fine line image with 100 .mu.m or less cannot be precisely reproduced at the printed material.
Moreover, there is a problem that development latitude is narrow. That is, the problem is that a width of the suitable development time is narrow and when it is out of the range, printing characteristics are markedly lowered.